Evaluation of an enclosed air-lift photobioreactor (ALPBR) for biomass and lipid biosynthesis of microalgal cells grown under fluid-induced shear stress

Abstract

An enclosed air-lift photobioreactor (ALPBR) is considered an efficient lab-scale bioreactor for microalgae cell growth and lipid biosynthesis. However, fluid-induced shear stress and mixing are two main factors that affect physiological metabolism in microalgal cell cultures. Herein, a 50-L ALPBR after being designed and manufactured was evaluated for microalgal suspension culture. Moreover, computational fluid dynamics (CFD) simulation was used to characterize the hydrodynamics of ALPBR. Specifically, two model microalgae species Chlorella vulgaris and Chlorella protothecoides were cultured with aeration rates at 0.1, 0.2, 0.3, 0.4 volume per volume per minute (vvm). The hydrodynamics of ALPBR while considering shear rate, volume average velocity, turbulent energy dissipation rate (EDR), air volume fraction, and mean air bubble diameter, etc. showed that aeration generated a very low shear stress. When C. protothecoides and C. vulgaris were cultured with various aeration rates, the maximum dry cell weights (DCW) of two microalgal species tested were 0.3 vvm and 0.2 vvm, while the maximum lipid contents of both species were about 15.0% of the DCW, which were achieved at an air flow rate of 0.2 vvm. Therefore, we concluded that the shear rate generated by aeration could play a vital role in the cell growth of microalgae grown in an air-lift photobioreactor.

Effect of Gas Holdup on CO2 Biofixation by Spirulina sp. in the 20-liter Airlift Bioreactor

“…Moreover, increasing in the gas velocity improved a better mixing in bioreactor so that microalgae could move faster toward the light, which caused a better light absorption hence, the microalgae had a better photosynthesis and growth rate. In a similar study (Ding et al 2021) which carried out in a 50 L airlift photobioreactor with 38 L working volume and 5.0% input CO2, at air flow rates of 8.0, 12.0 the maximum dry cell of Chlorella protothecoides was achieved 0.24 g/L at 12.0 L/min and 300 h. They found that when the aeration rate was 8.0 L/min, the final biomass concentration of C. protothecoides reached 0.32 g/L at 500h, in contrast, in the aeration rate of 12.0 L/min for 300 h the dry weight was 0.24 g/L, which was higher than at 8.0 L/min and 300 h. Also, it was observed that the maximum biomass concentration of Spirulina platensis in the batch cultivation and continuously preharvesting (different operation mode) was 3.383 and 4.893 g L -1 after 11 days' cultivation respectively (Liu et al 2018). whereas this experiment carried out (5% CO2 in air) and 100 mL min -1 gas flow rate in one hollow cylinder with total working volume 1L and in Zarrounk culture medium.…”

“…Besides, gas holdup is affected by superficial gas velocity, which is called gas flow. By increase gas flow into the reactor, gas holdups in riser and downcomer increase as a result, liquid circulation velocity rise and bubbles which move smoothly in the downcomer and riser and they have spherical shape, despite keeping their shape, larger bubbles move faster and descend to the downcomer which lead to the better light accessibility to microalgae (Ding et al 2021, Rengel et al 2012. However, in higher air flow rates, as liquid circulation velocity increases microalgae cannot stay a long time in downcomer so that they are not able to utilize light efficiently (Sánchez Mirón et al 2000).…”

Investigation of Hydrodynamic Parameters in an Airlift Photobioreactor on CO2 Biofixation by Spirulina sp.

“…They confirmed that total gas holdup increases linearly with the inlet gas velocity in the range of velocity investigated. The reason for this was the change in the shape and nature of gas bubbles at a higher velocity of the inlet gas [30,47,48]. As gas velocity increased, bubble size also increased (larger bubbles were released from the sparger at the bottom), but as these large bubbles rose in the liquid column, they quickly broke down into small bubbles; as a result, the number of smaller bubbles increased and the mean bubble diameter decreased, which is presented in Table 4; also the accumulation of large bubbles in the center and small ones along the walls of the reactor was observed.…”

“…As the gas flow rate increases, the gas holdup in the riser and downcomer increases, and as a result, the liquid circulation and bubble velocities increase. Larger bubbles move faster and descend to the downcomer, which results in better light absorption by cyanobacteria [29,30]. However, as the liquid velocity increases at higher gas flow rates, cyanobacteria do not remain in the downcomer long enough to utilize light efficiently [31] and may suffer from shear stress.…”

Microalgae cultivation: photobioreactors, CO2 utilization, and value‐added products of industrial importance

“…Vertical-column/airlift photobioreactors Vertical-column/airlift PBRs are low in cost, available in various designs and scales, and have been tested for the easy cultivation of monoalgal cultures under controlled and aseptic conditions. [49][50][51][52][53][54] Vertical tubular PBRs consist of a cylinder made of transparent glass or PVC with a sparger located usually at the bottom and external or internal illumination. This typical reactor is called a bubble column reactor.…”